Subprograms

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Subprograms

• Jianhua Yang
• Department of Math and Computer Science
• Bennett College

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Outline

• Fundamentals of Subprograms
• Design issues
• Local Referencing Environments
• Parameter-Passing Methods
• Parameters that are subprogram names
• Overloaded Subprograms
• Generic Subprograms
• Design Issues for functions
• User-defined overloaded Operators
• Coroutines

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9.1 Fundamentals of Subprograms

• Characteristics
• Single entry point
• Caller suspended
• Return to caller

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1. Basic Definition

• A subprogram definition describes the interface
  to and the actions of the subprogram abstraction.
• Procedure
• Function

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Subprogram header

• 1. it is a subprogram with a type
• 2. specifying its name
• 3. specifying a list of parameters

Fortran Subroutine Adder (parameters) Ada
procedure Adder (parameters) C-base
language void adder (parameters)
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Parameter profile

• It contains the number, order, and types of its
  formal parameters.

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2. Parameters

• Formal parameters
• Actual parameters
• Positional parameters
• Keyword parameters

Sumer (Length gt My_Length, List gt
My_Array, Sum gt My_Sum)
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Default value for formal parameters

• Ada function header
• function Compute_Pay (IncomeFloat
• Exemptions Integer 1
• Tax_Rate Float) return Float

Call this function Pay Compute_Pay(2000.0,Tax_
Rategt0.15)
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C example

• float compute_pay (float income, float tax_rate,
  int exemptions 1)

The default parameters must appear last.
Call this function pay compute_pay(2000.0,
0.15)
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3. Procedures and Functions

• They are two distinct categories of subprograms

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Procedure

• Procedures are collections of statements that
  define parameterized computations.

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Interaction between procedure and its caller

• 1. Visible variables in both a procedure and the
  caller can be changed by the procedure.
• 2. if a procedure has formal parameters that
  allow the transfer of data to the caller, those
  parameters can be changed.

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Function

• Different from a procedure, it modifies
• Neither its parameters
• Nor any variables defined outside the function.

• Functions are called by appearances of their
  names in expressions, along with the required
  actual parameters.
• Return a value to the calling code (caller).

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Function example in C

• float power (float base, float exp)
• Called with
• result 3.4 power(10.0, 3)

Functions can be used as a procedure
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9.2 Design Issues for Subprograms

• Parameter-passing method
• Type check
• Local variables allocation statically or
  dynamically
• Nested definition
• Subprogram as parameter
• Overload
• Generic.

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9.3 Local referencing environments

• Variables that are defined inside subprograms are
  called local variables
• The scope is usually the body of the subprogram
  in which they are defined.

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Local variables

• Static
• Stack-dynamic

They are bound to storage when the subprogram
begins execution and are unbound from storage
when that execution terminates.
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Stack-dynamic

• Ad.
• Flexibility
• Space share
• Da.
• Time cost
• Indirect access
• Non history-sensitive

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Static

• Ad.
• Efficiency
• No run-time overhead
• Da.
• Inability to support recursion
• Space share problem

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Example

• int adder (int list, int listlen)
• static int sum0
• int count
• for(count0 count lt listlen count)
• sum listcount
• return sum

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9.4 Parameter-Passing Methods

• Parameter-passing methods are the ways in which
  parameters are transmitted to and/or from called
  subprograms.

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1. Semantics Models of Parameter Passing

• In mode
• Out mode
• In-out mode

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2. Implementation models of parameter passing

• Pass-by-value
• Pass-by-result
• Pass-by-value-result
• Pass-by-reference
• Pass-by-name

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1) pass-by-value
Callee Void sub (int x, int y, int z)
Caller sub (a, b, c)

In mode

Out mode


Inout mode
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Disadvantages

• 1. space cost
• 2. large data transfer cost too much time

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2) pass-by-result

• It is an implementation model for
  out-mode-parameters.
• The same disadvantages as 1).

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3) Pass-by-value-result

• This one is in effect a combination of
  pass-by-value and pass-by-result.
• Also called pass-by-copy.
• The same disadvantages as the previous ones.

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4) Pass-by-reference

• Rather than copying data values back and forth,
  this method transmits an access path, usually
  just an address, to the called subprogram.
• Its advantage is its efficiency in terms of time
  and space.
• Its disadvantages
• 1. indirect accessing
• 2. inadvertent errors
• 3. aliases problem

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Example
void sub(int param)

• void main()
• int global
• sub(global)

Memory block
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5)pass-by-name

• skip

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3. Multidimensional Arrays as Parameters

• void fun (int matrix10)
• Void main()
• int mat510
• fun (mat)

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Examples of parameter passing
void swap1( int a, int b) int
tempa ab btemp
void swap2( int a, int b) int
tempa ab btemp
swap2(c, d)
swap1(c, d)
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9.5 Parameters that are subprogram names

• In C, and C, function name can not be used as a
  parameter
• But we can use a pointer point to a function.

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9.6 Overloaded Subprograms

• Overloaded operator is one that has multiple
  meanings.
• Overloaded subprogram is a subprogram that has
  the same name as another subprogram in the same
  referencing environment.
• The meaning of a call to an overloaded subprogram
  is determined by the actual parameter list.

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example

• void fun (int, int)
• void fun (float)
• void fun()

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9.7 Generic subprograms

• Purpose is to increase the reusability of
  software
• A generic or polymorphism takes parameters of
  different types on different activations.

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1. Generic Functions in C

• Generic functions in C have the descriptive
  name of template functions.
• General form
• template lttemplate parametersgt
• A template parameter has one of the following
  forms
• class identifier
• typename identifier

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Example

• template ltclass Typegt
• Type max (Type first, Type second)
• return first gt second ? First second

int max (int first, int second) return first gt
second ? first second
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Generic sort example
template ltclass Typegt void generic_sort ( Type
list, int len) int top, bottom Type
temp for (top0 topltlen-2 top) for
(bottom top 1 bottom ltlen -1 bottom) if
(listtop gt listbottom) temp
listtop listtop listbottom listbo
ttom temp
float flt_list100 generic_sort(flt_list,
100)
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9.8 Coroutines

• A special kind of subprogram
• Might have multiple entry-points
• The invocation of a coroutine is called a resume
  rather than a call
• Only one coroutine actually executes at a given
  time
• Quasi-concurrency

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example
A
B
resume B resume B resume B
resume A resume A
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Summary

• Subprogram definitions
• Local referencing environments
• Parameter passing
• Overloaded subprograms
• Generic subprograms
• Coroutines.